Moonday: Tides of DOOM

Over the course of many Moondays, we have explored a lot of the satellites in the (relatively) heavily explored regions of the Solar System: Mars, the larger moons of Jupiter, a lot of Saturn’s wonderfully odd moons, and of course our own Moon. To put it mildly, we’re nowhere close to covering every moon, even if we neglect the tiny asteroid-like moons we’ve never even photographed in detail.

Triton, Neptune’s largest moon, as seen by Voyager 2. The surface is frozen nitrogen, along with water, methane, and ammonia ice, renewed by cryovolcanism: ice volcanoes analogous to what we see on Saturn’s moon Enceladus.

However, as several friends and readers have reminded me, I’ve neglected one significant satellite: Triton, Neptune’s largest moon. Triton is the seventh largest moon in the Solar System (after the Galilean moons, Titan, and Earth’s Moon). It’s also overwhelmingly the largest moon orbiting Neptune, having nearly 300 times more mass than all other Neptunian moons combined. Unlike many other large moons in our Solar System, Triton has been visited just once, by the Voyager 2 spacecraft which flew past in 1989. (Since I paid a lot of attention to such things at the time, I feel old just typing that. It doesn’t feel like 23 years have passed!) Only about 40% of Triton’s surface has been imaged as a result, and as far as I know, no mission to Neptune and its moons is planned for the foreseeable future.

A detailed Voyager 2 image of cantaloupe terrain on Triton. This strange feature is free of craters, but exactly why it appears the way it does is mysterious.

Triton is fairly smooth, having few impact craters and no significant mountains or valleys. In this way, it resembles Jupiter’s Europa, Saturn’s Enceladus, and other ice moons. Another way it resembles Enceladus is in its activity: Triton sends out plumes from cryovolcanoes, volcanoes that spew icy material instead of molten lavas. These cryovolcanoes erase the craters from meteor impacts, but also depositing dusty material from Triton’s interior onto the surface. Another interesting feature is the so-called “cantaloupe terrain”, which really does resemble the skin of a melon. While the cantaloupe terrain may also be associated with cryovolcanism, its exact origin is mysterious.

Despite Triton’s wonderful appearance, its orbit is even more wonderful, and reflects the complex and fascinating history of the Solar System. Triton is an irregular satellite, along with Saturn’s moon Phoebe, meaning in this case that it orbits in reverse to Neptune’s rotation. All the other large moons—including the Moon—orbit in the same direction as their planet’s rotation, which reflects a common origin for host and satellite. Triton, on the other hand, was probably captured by Neptune’s gravity—in which case originated elsewhere, possibly in the Kuiper Belt. This claim is bolstered by Triton’s chemical similarity to Pluto, the first-observed and most famous Kuiper Belt object: Pluto also has nitrogen frost on its surface, and a similar total density.

Triton’s size compared to Earth and the Moon.

Triton’s interesting orbit doesn’t stop with the direction. It also is highly inclined compared to Neptune’s equator, meaning its orbit takes it over the northern and southern hemispheres alternately (while regular satellites are more nearly aligned with their hosts’ equators). Even more strangely, its orbital inclination changes in time, becoming more or less tilted. As we’ve seen with many other moons, this is a tidal effect: the complex way in which Neptune’s gravity acts on Triton has not only slowed the moon’s rotation, but also has torqued its orbital motion.

Will Neptune have a glorious set of rings in the future after it destroys Triton? (Actually, the real reason to post this picture is just because I like Neptune.)

Tidal forces also have signed Triton’s death warrant: the moon orbits very close to Neptune, meaning the pull on the near side of the moon is significantly higher than the far side. While this is a common phenomenon and the reason why the Moon always presents the same face to Earth, Triton is close to the Roche limit, where the tidal forces are comparable to the internal forces holding it together. While no one knows exactly when or how this will happen, at some point in the future Neptune’s gravity will break Triton apart—possibly giving Neptune a glorious set of rings to rival Saturn. (As with Jupiter and Uranus, Neptune does have rings, but they are darker and less dramatic than Saturn’s.)

Alas, the destruction of Triton is unlikely to happen in my lifetime (most estimates I’ve seen place the Doom of Triton billions of years in the future)…but I still hope for it. The sacrifice of a beautiful moon seems small compared to what we’d learn from watching tidal disruption in action, and the aesthetic beauty of another glorious ring system also seems worth the loss. The end of Triton would be a beautiful death.